Understanding language is one of the most fundamental human cognitive abilities. It plays an important role in normal development, and is the major means for acquiring information in many domains. A number of psychological disorders (e.g., schizophrenia) can disrupt the normally impressive functioning of this system, greatly exacerbating the negative consequences of these disorders. Psycholinguists have made significant progress in clarifying the structures and processes that underlie language comprehension. Despite this progress, much remains to be learned about how words are represented in a person's "mental lexicon". Critical remaining issues in this area revolve around questions of lexical representation, and lexical access. Theories must specify how the presentation of a spoken word leads to a particular lexical representation becoming activated, and what the effects of such activation are: What effect does one active lexical representation have on others, and on units at other levels of representation? The current proposal includes a large set of theoretically-driven empirical studies of lexical activation. The empirical investigations are organized into three interlocking groups of experiments. One set of studies examines how phonetic variation can affect lexical access, and can even change the lexical representations themselves. Such variation can have surprisingly powerful effects on how words are represented in the lexicon. A second set of studies examines even more fundamental changes in the lexicon: How do adults add new items to their mental lexicons, and what are the consequences of such changes? The third set of experiments investigates the dynamics of lexical competition: How does the activation of one lexical item affect the activation levels of other lexical entries? The product of the proposed research will be a much better understanding of the architecture of the system that accomplishes language comprehension. These studies will provide a much more detailed picture of how the mental lexicon changes over both the short term, and the longer term. Such an understanding is critical to our understanding language processing. In turn, because language is such a fundamental cognitive ability, progress in describing language processing will enhance our understanding of human cognition, under both normal and disordered conditions.